DataGrid Need A Horizontal ScrollBar and Resizing Capability

The problem is that the width of the DataGrid is bound to the width of the ScrollViewer

<DataGrid Width={Binding ActualWidth, RelativeSource={RelativeSource AncestorType={x:Type Grid}}}">

The DataGrid is never going to be wider than the ScrollViewer so the scrollbar will not be enabled.

Setting HorizontalAlignment to 'Stretch' will achieve the layout that I think you are looking for and allow scrolling. So the DataGrid element ought to be:

 <DataGrid HorizontalAlignment="Stretch">
                <DataGrid.Columns>
                    <DataGridTextColumn Header="H1" Width="40" MinWidth="35"/>
                    <DataGridTextColumn Header="H2" Width="*" MinWidth="47"/>
                </DataGrid.Columns>
            </DataGrid>

Here's what I came up with by making use of VisualTreeHelper.

Create a method that can search the Visual Tree for a child of a particular type. I found a method for looking for a parent (the reverse direction) that I was able to modify. https://stackoverflow.com/a/636456/1640271.

public static T FindVisualChild<T>(DependencyObject parent)
    where T : DependencyObject
{
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
    {
        DependencyObject childObject = VisualTreeHelper.GetChild(parent, i);

        if (childObject == null) return null;

        if (childObject is T obj)
        {
            return obj;
        }
        else
        {
            return FindVisualChild<T>(childObject);
        }
    }

    return null;
}

My code behind included creating one constant, one class variable, and using two Window event triggers.

Constant

(imposes a constraint on the Window width to ensure that at least some portion of the second columns can always be seen).

private const double _item2MinViewableWidth = 100;

Variable

(used to capture the Grid from the Visual Tree that is inside the ScrollView of the DataGrid).

private Grid _innerGrid;

Events

private void Window_ContentRendered(object sender, EventArgs e)
{
    _innerGrid = UIHelper.FindVisualChild<Grid>(dataGrid);

    item2Column.MinWidth = _item2MinViewableWidth;

    item1Column.Width = new DataGridLength(1, DataGridLengthUnitType.Auto);
    item2Column.Width = new DataGridLength(1, DataGridLengthUnitType.Auto);

    MaxWidth = ActualWidth;

    _innerGrid.MinWidth = item1Column.ActualWidth + _item2MinViewableWidth;
    _innerGrid.Width = item1Column.ActualWidth + _item2MinViewableWidth;

    UpdateLayout();

    var mainGridInitWidth = mainGrid.ActualWidth;
    MinWidth = ActualWidth;
    _innerGrid.Width = Double.NaN;

    mainGrid.Width = mainGridInitWidth;
}

private void Window_SizeChanged(object sender, SizeChangedEventArgs e)
{
    if (e.WidthChanged && _innerGrid != null)
    {
        var widthDetla = e.NewSize.Width - e.PreviousSize.Width;
        mainGrid.Width += widthDetla;
    }

    if (Math.Abs(ActualWidth - MaxWidth) < 1)
    {
        dataGrid.HorizontalScrollBarVisibility = ScrollBarVisibility.Hidden;
    }
    else
    {
        dataGrid.HorizontalScrollBarVisibility = ScrollBarVisibility.Auto;
    }
}

Here's my XAML for good measure:

<Window
x:Class="WPFSandbox.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:WPFSandbox"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
Title="MainWindow"
Height="450"
ContentRendered="Window_ContentRendered"
ResizeMode="CanResize"
SizeChanged="Window_SizeChanged"
SizeToContent="Width">

<Grid x:Name="mainGrid">
    <DataGrid x:Name="dataGrid"
        AutoGenerateColumns="False"
        CanUserResizeColumns="False"
        HeadersVisibility="Column"
        IsReadOnly="True"
        ItemsSource="{Binding MyItems}">
        <DataGrid.ColumnHeaderStyle>
            <Style TargetType="{x:Type DataGridColumnHeader}">
                <Setter
                    Property="SeparatorVisibility"
                    Value="Hidden" />
            </Style>
        </DataGrid.ColumnHeaderStyle>
        <DataGrid.Columns>
            <DataGridTextColumn x:Name="item1Column"
                Binding="{Binding Item1}"
                Header="Item1" />
            <DataGridTextColumn x:Name="item2Column"
                Binding="{Binding Item2}"
                Header="Item2" />
        </DataGrid.Columns>
    </DataGrid>
</Grid>

Hopefully, this is in line with what you need.